TRIGGER

I want SQL TRIGGER that can update stock table by subtracting the inserted value of Product table from the Stock table. Please it is the Quantity column of the Product table I want to subtract from the Qauntity_Instock on the Stock table any time values are inserted into the Product table. Below is the table codes

Welcome to the forums!
This suspiciously looks like a homework assignment, which you really should solve yourself. For a start, search for "INSERTED" and "DELETED". The rest is not all that hard to figure out.

For now this is what I have done but when I insert values into the
transaction table is not able to deduct the inserted values from the
Stock table. So please modify the code for me and is not home work.
Thanks.

Okay, I'm not entirely sure, that your trigger fires on the correct event. I would have assumed that you would want an AFTER UPDATE trigger or something like that. Also, your trigger can handle only one product at a time, which may or may not be what you want. If you need to handle multiple products at once, you need something like this: